In geomagnetism, the angle between the geographical meridian at a place and the magnetic meridian indicated by a freely suspended magnet is known by which term?

Introduction / Context:

Earth has both a geographical reference frame based on the rotation axis and a magnetic reference frame based on its magnetic field. At most locations, the magnetic north direction does not perfectly coincide with true geographical north. The difference between these directions is important in navigation and surveying. This question asks for the correct term used to describe the angle between the geographical meridian and the magnetic meridian at a given place.

Given Data / Assumptions:

• A geographical meridian is the line pointing to true north south on a map based on Earth rotation axis.
• A magnetic meridian is the line indicated by a freely suspended magnetic needle pointing toward magnetic north south.
• We are asked to name the angle between these two meridians at a place.
• Options include angle of declination, angle of dip, angle of inclination, and azimuth.

Concept / Approach:

Angle of declination is defined as the horizontal angle between true north (geographical meridian) and magnetic north (magnetic meridian). Angle of dip refers to the angle made by Earth's magnetic field with the horizontal plane, measured vertically. Angle of inclination is sometimes used interchangeably with dip in magnetism, and azimuth is a general term for the horizontal angle of a line measured from a reference direction, usually true north, but not necessarily specific to magnetic versus geographical meridians.

Step-by-Step Solution:

Verification / Alternative check:

Physics and geophysics textbooks define magnetic declination as the angle in the horizontal plane between the direction of the geographic meridian and the magnetic meridian. Navigation manuals for compasses also refer to declination charts that tell users how to correct compass readings to obtain true bearings.

Why Other Options Are Wrong:

Angle of dip (or inclination) is measured in a vertical plane and indicates how much the field lines plunge into the Earth, not the horizontal difference between north directions.

Angle of inclination, in the context of magnetism, again refers to the tilt of the magnetic field with respect to the horizontal, not to the difference between geographic and magnetic meridians.

Azimuth is a general term for the horizontal angle of any direction measured from true north, often used in surveying, and is not restricted to describing the separation between geographic and magnetic meridians.

Common Pitfalls:

Students sometimes confuse declination and dip because both involve magnetic directions. A good way to remember is that declination deals with horizontal difference between two north directions, while dip deals with vertical tilt of the magnetic field.

Final Answer:

The angle between the geographical meridian and the magnetic meridian is called the angle of declination.